The present invention relates to self-actuated scram systems for dropping neutron absorbing poisons into the core of a nuclear reactor, and in particular to systems responsive to the rate of pressure drop associated with a loss of forced liquid coolant flow.
In all kinds of nuclear power reactors, a reactor coolant flows through the power producing core of the reactor in order to remove the heat generated therein. If the coolant flow rate becomes too low in proportion to the power level of the core, a dangerous condition occurs wherein the core will become so hot that damage to the fuel is likely.
The plant protective system has instrumentation designed to sense a flow rate that is too low, and to drop (scram) safety poison rods into the reactor core, thereby terminating the power produced therein. Particularly in reactors designed to operate with a fast neutron energy spectrum (fast reactors) and with a high power density, it is essential that the safety poison be inserted very quickly upon a rapid drop in the coolant flow rate. It is desirable in this kind of reactor to have a backup method of dropping poison into the reactor core that does not rely on instrumentation, but rather is self-actuated by a rapid drop in the coolant flow rate. It is important, however, that the backup system not initiate a scram in response to the normal power-dependent changes in coolant flow.
The normal flow rate through a liquid-metal cooled reactor is approximately proportional to the power level, and the normal coolant pressure at any location in the reactor is allowed to vary with the flow. A rapid loss of forced flow will produce an immediate drop in the coolant pressure throughout the reactor. The rate of pressure change associated with loss of flow incidents is typically known from calculation or measurements, but this knowledge has not previously been used to achieve self-actuated scram on loss of liquid coolant flow.
The prior art contains self-actuated scram systems for responding to rapid changes in the pressure of a gaseous reactor coolant, where the coolant pressure itself rather than the flow rate is the most important safety parameter. One prior art device responsive to the rate of change of pressure in a gas-cooled reactor is described in British Pat. No. 872,092 issued to S. Baldwin et al, on July 5, 1961. This device has a combined sensor chamber and actuating bellows filled with gas that is in fluid communication, through a small orifice in the chamber, with the gas coolant flowing outside the chamber. A large rate of change of pressure drop causes the actuating bellows to expand due to the inability of the pressure within the chamber to rapidly equilibrate through the orifice. The bellows expansion actuates a release mechanism to drop poison material into the core. If a Baldwin-type device were placed in a liquid coolant environment, the presence of liquid both inside and outside the chamber would produce at best only a very small actuating bellows motion, even for a large rate of pressure drop. In a liquid environment the lack of fluid compressability would preclude operability of the device. This is particularly true in a liquid metal-cooled reactor where the maximum rate of change in pressure during the most severe loss of flow accident is only about 20 psi per second.
The prior art known to the Applicants is unsatisfactory for use with a liquid coolant, and does not suggest modifications for improving the performance of the prior art in a liquid-cooled reactor.